virtio: pick simple Rx/Tx
virtio: pick simple Rx/Tx

simple rx/tx func is chose when merge-able rx is disabled and user
specifies single segment and no offload support.

Signed-off-by: Huawei Xie <3b3be4428106761300a2ee559c9f71ecdb091553@intel.com>
Acked-by: Jianfeng Tan <4339d9ba3a87a0d192e405945005ff86667f7b0c@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk